Technological advances in recent years have made it easier for individuals and groups in geographically disperse societies to be interconnected through physical travel and communication systems. Major advances in the telecommunications infrastructure have been developed and are continuously evolving to meet the needs of people who regularly travel, communicate, and do business internationally. For example, satellite-based global communication networks have arisen to serve the needs of global travelers and communicators. One such network, first activated in 1998, is the Iridium® commercial system. The Iridium® commercial system is a satellite-based global digital communication network designed to provide wireless communications through hand-held devices located anywhere near or on the surface of the Earth.
FIG. 1 illustrates a highly simplified diagram of a satellite-based communication network 20, dispersed over and surrounding Earth through the use of orbiting satellites 22 occupying orbits 24. Network 20 uses six polar orbits 24, with each orbit 24 having eleven satellites 22 for a total of sixty-six satellites 22. As such, network 20 exemplifies the Iridium® commercial system.
Satellites 22 communicate with radio communication individual subscriber units (ISU's) 26 over subscriber links 28. In addition, satellites 22 communicate with earth terminal/gateway systems 30, which provide access to a public switched telephone network (PSTN) 32 or other communications facilities, over earth links 34. Earth terminal/gateway systems 30 (referred to hereinafter as gateways 30) relay data packets (e.g., relating to calls in progress) between ISU's 26 and the PSTN 32 to other communication devices, such as a wireline telephone 36. Satellites 22 also communicate with other nearby satellites 22 through cross-links 40. For simplicity of illustration, only one each of ISU's 26, gateways 30, and a wireline telephone 36 are shown in FIG. 1.
With the exemplary constellation of sixty-six satellites 22, at least one of satellites 22 is within view of each point on the Earth's surface at all times, resulting in full coverage of the Earth's surface. Any satellite 22 may be in direct or indirect data communication with any ISU 26 or gateway 30 at any time by routing data through the constellation of satellites 22. Accordingly, communication network 20 may establish a communication path for relaying information through the constellation of satellites 22 between any two ISU's 26, or between ISU 26 and gateway 30.
Network 20 may accommodate any number, potentially in the millions, of ISU's 26. Subscriber links 28 encompass a limited portion of the electromagnetic spectrum that is divided into numerous channels, and are preferably combinations of L-Band frequency channels. Subscriber links 28 may encompass one or more broadcast channels 42, that ISU's 26 use for synchronization and message monitoring, and one or more acquisition channels 44 that ISU's 26 use to transmit messages to satellites 22. Broadcast channels 42 and acquisition channels 44 are not dedicated to any one ISU 26 but are shared by all ISU's 26 currently within view of a satellite 22.
Subscriber links 28 also include wireless traffic channels 46, also known as voice channels. Traffic channels 46 are two-way channels that are assigned to particular ISU's 26 from time to time for supporting real-time communications. Each traffic channel 46 has sufficient bandwidth to support a two-way voice communication. For example, each of traffic channels 46 within the Iridium® network are capable of approximately 2.4 kilobits/second (kbps) raw data throughput.
In a variety of applications, such as military, medical, humanitarian, distance learning, and others, the capability to transmit digital imagery and real-time video is highly desirable. A video coder/decoder (i.e., codec) is typically employed for the transmission of the digital imagery. A video codec compresses digital video data into an encoded form according to a given video file format or streaming video format, to facilitate transmission, storage, or encryption.
Referring to FIGS. 2-3, FIG. 2 shows a block diagram of a conventional video coder 48, and FIG. 3 shows a block diagram of a standard differential pulse code modulation (DPCM) loop 50 of video coder 48. Video coder 48 includes a motion estimation/compensation and DPCM prediction function 52, followed by a spatial image transform function 54, a quantizer function 56, and an entropy coder function 58. In general, video coder 48 receives successive video frames 60 and compresses video frames 60 to facilitate the transmission of compressed video frames 60 over a transmission channel 62.
As known to those skilled in the art, successive video frames 60 may contain the same objects (still or moving). Motion estimation/compensation and DPCM prediction function 52 examines the movement of objects in an image sequence to try to calculate vectors representing the estimated motion. For interframes, which are frames coded with reference to previous frames, motion estimation is used to predict the current video frame 60 from the previous one. Once the current video frame 60 has been predicted based on the calculated motion information, an error frame is generated by using DPCM loop 50. For intraframes, which are frames coded without reference to previous frames, the motion estimation and DPCM prediction operations are omitted, and the frame content is coded.
Following function 52, image transform function 54 concentrates the energy of video frames 60 into a smaller region. Commonly used image transforms are block-based ones, such as the discrete cosine transform (DCT), and the subband transforms, such as the discrete wavelet transform (DWT). After the transform, the transform coefficients are quantized at quantizer function 56 and encoded at entropy coder function 58. The entropy coded transform coefficients are then transmitted via transmission channel 62 to a decoder.
In a vast number of regions throughout the Earth, there exists little or no infrastructure capable of effective communication of digital imagery and video. Consequently, techniques are evolving to utilize satellite-based networks, such as the Iridium® commercial system, to transmit digital imagery and video, in addition to large data files and voice communications. Such a technique is described in the aforementioned related invention, “System And Method For Satellite-Based Transmission Of Signals Using Multiple Channels,” U.S. patent application Ser. No. 10/404,791. The technique extends the capability of voice optimized traffic channels, within a wireless communication system, for the transmission of data and video.
Unfortunately, transmission of digital imagery and video over low-bit-rate, wireless links, such as traffic channels 46 is extremely problematic due to limited channel bandwidth. In addition to the limited available bandwidth, wireless traffic channels 46 have a high probability of error due to latency, fading effects, and/or channel drop out. Conventional video coders, such as video coder 48, are incapable of effectively interfacing with satellite-based communication networks to facilitate the transmission of digital imagery and real-time video under the conditions of limited bandwidth, latency, fading effects, and/or channel drop out. Accordingly, what is needed is a system and method for facilitating the transmission of video in a satellite-based communication network that operate over multiple wireless channels, and account for the latency, fading effects, and limited available bandwidth inherent in such a network.